This invention relates to alternating current plasma systems and more particularly to an electrode assembly for use in such systems of the radio frequency plasma type for continuously producing photovoltaic devices by depositing successive amorphous-silicon alloy semiconductor layers on a substrate in each of a plurality of deposition chambers. The composition of each amorphous layer is dependent upon the particular process gases introduced into each of the deposition chambers. The gases introduced into the deposition chambers are carefully controlled and isolated from the gases introduced into adjacent deposition chambers. More particularly, the deposition chambers are operatively connected by a relatively narrow gas gate passageway (1) through which the web of substrate material passes; and (2) adapted to isolate the process gases introduced into adjacent deposition chambers.
To facilitate the deposition of the amorphous silicon alloy materials onto the substrate, each chamber includes an electrode assembly having an electrode which receives alternating current power and a gas inlet for receiving the process gases. The alternating current power, in the form of radio frequency power, is coupled to the electrode to cause a plasma to be formed from the process gases to facilitate the deposition of amorphous silicon alloy materials onto a substrate. To provide coupling between the radio frequency power source and the electrode, a tuning network is placed between the source and electrode. The tuning network matches the output impedance of the source to the input impedance of the electrode.
In order to confine the plasma region between the electrode and the substrate and to prevent the formation of a plasma on the side of the electrode opposite the substrate, a shield, formed from conductive material, is disposed on the side of the electrode opposite the substrate. A bottom wall of the shield has a surface area at least as large as the surface area of the electrode and is closely spaced therefrom by a distance equal to or less than the dark space dimension to prevent a plasma from being formed between the shield and the electrode. Unfortunately, because the shield is coupled to ground potential, and by virtue of the close spacing between and the large surface areas of the electrode and the shield, a large capacitance results therebetween. This large capacitance makes it difficult to efficiently couple the radio frequency power to the electrode.
Because the electrode input impedance includes a large capacitive reactance component, large tuning networks are necessary to provide the required compensation or tuning. Also, such high capacitances result in high circulating currents, on the order of hundreds of amperes, within the tuning networks resulting in excessive I.sup.2 R losses and heat problems attendant therewith. Heating problems of such magnitude have occurred in the past that water cooling has been found necessary. Also, the high circulating currents require the tuning components to be of high current rating, adding to their size and expense. It is to the end of reducing the electrode assembly capacitance that the present invention is directed.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices which are, in operation, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques that have (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) provide high quality electronic properties. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is increased.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in U.S. Pat. No. 4,400,409 issued Aug. 23, 1983 to M. Izu et al. for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; U.S. Pat. No. 4,410,538, issued Oct. 18, 1983 to M. Izu et al. for Continuous Amorphous Solar Cell Production System; U.S. Pat. No. 4,438,723 issued Mar. 27, 1984 to V. D. Cannella et al. for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other chambers to prevent the back diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which unidirectional gas flow is established and through which an inert gas may be "swept" about the web of substrate material.
In the previously mentioned patent applications, deposition of the amorphous silicon alloy materials onto the large area continuous substrate is accomplished by glow discharge decomposition of the process gases. To that end, each deposition chamber includes an electrode assembly for establishing the glow discharge plasma. The plasmas are maintained by an electrode which is coupled to an alternating current power source such as a radio frequency generator operating at a frequency of, for example, 13.56 megahertz. To effectively transfer the radio frequency power from the generator to the electrode, a tuning network is disposed between the generator and electrode. The tuning network matches the output impedance of the generator, which is substantially purely resistive, to the input impedance of the electrode. As previously mentioned, the input impedance of prior electrodes include a large capacitive reactance component making matching difficult. The present invention provides an electrode assembly wherein the capacitive reactance component of the electrode input impedance is substantially reduced.
It is therefore a general object of the present invention to provide a new and improved electrode assembly for use in an alternating current plasma system wherein alternating current power is transferred from a power source to an electrode by a tuning network.
It is a more particular object of the present invention to provide a new and improved electrode assembly for use in a radio frequency plasma continuous processing system for making photovoltaic devices.
It is another object of the present invention to provide such an electrode assembly which has a substantially reduced input capacitance to more readily facilitate the transfer of alternating current power to the electrode of the electrode assembly.